Design a Highly Reactive Trifunctional Core Molecule To Obtain

Article pubs.acs.org/Macromolecules

Design a Highly Reactive Trifunctional Core Molecule To Obtain Hyperbranched Polymers with over a Million Molecular Weight in One-Pot Click Polymerization Xiaosong Cao,† Yi Shi,† Xiaofeng Wang, Robert W. Graff, and Haifeng Gao* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: We present the first one-pot one-batch synthesis of hyperbranched polymers with over a million molecular weight and low polydispersity in the coppercatalyzed alkyne−azide cycloaddition (CuAAC) polymerization of AB2 monomer. In contrast to the traditional triazido core molecule that failed to produce high molecular weight polymer, a novel tris-triazoleamine-based B3 molecule was designed, which complexed with CuI catalyst and activated the azido reactivity in both B3 and B3-containing polymers. The polymerization demonstrated linear increase of polymer molecular weights with the feed ratio of [AB2]0:[B3]0 with no need of slow monomer addition. At the same time, a higher ratio of [CuI]0:[B3]0 showed no influence on molecular weight but significantly increased the polymerization rate and produced hyperbranched polymers with higher degree of branching. The one-pot polymerization using [AB2]0:[B3]0:[Cu]0 = 2700:1:10 produced hyperbranched polymers with molecular weight Mn = 1.01 × 106 and polydispersity Mw/Mn = 1.05 in 4 h.

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INTRODUCTION Hyperbranched polymers, as an important category of soft nanomaterials, have received considerable interest and found various applications in catalysis, biomaterials, microelectronics, and nanomedicines, due to their three-dimensional structure, cavernous interior, and large number of peripheral functionalities.1−8 In comparison to dendrimers that require tedious multistep synthesis and inefficient chromatographic purification, hyperbranched polymers are generally prepared in an effortless one-pot polymerization of ABm (m ≥ 2) monomer9−18 or AB* inimer (containing initiator fragment B* and monomer vinyl group A in one molecule).19−28 However, the random bimolecular (e.g., monomer−monomer, polymer− polymer, and monomer−polymer) reactions in these one-pot synthesis significantly compromise the structural control of hyperbranched polymers and result in polymer product with extremely high polydispersity, which undermines the physical properties of hyperbranched polymers.29−31 One-pot synthesis of hyperbranched polymers with both high molecular weight and low polydispersity is challenging.32−35 The use of multifunctional core molecule Bf ( f ≥ 2) in the polymerization of ABm monomers and AB* inimers has been demonstrated in both simulations36,37 and experiments38−42 to decrease the polydispersity and moderately increase the molecular weights of hyperbranched polymers. In order to quickly incorporate all added Bf molecules into the polymers and selectively favor polymer−monomer reactions,40,43 two strategies have been developed in the literature © XXXX American Chemical Society

using (1) slow addition of monomers into a dilute solution of Bf core40,41 and (2) special monomer/core pairs to achieve high reactivity of functional groups on core-containing polymers.44,45 In particular, the latter method reported by Yokozawa represented the first one-pot chain-growth polymerization of AB2 monomers. However, low molar ratios of monomers to Bf cores (i.e., 106), low polydispersity (Mw/Mn < 1.1), and tunable degree of branching (DB = 0.65−0.86) using the one-pot one-batch CuAAC polymerization of AB2 monomer in the presence of a novel trifunctional B3 core molecule (Scheme 1). The specially designed B3 molecule carried a tris-triazoleamine55 moiety that could complex with CuI to achieve higher reactivity of azido groups in the B3 core and B3-containing polymers than those in AB2 monomers. This AB2/B3 system thus exhibited fast consumption of B3 molecules and favored polymer−monomer reactions over monomer−monomer reactions, resulting in a linear increase of polymer molecular weight versus monomer conversion.

RESULTS AND DISCUSSION

The AB2 monomer that contained one alkynyl group and two azido groups was prepared (Scheme 1), in which the threecarbon spacer between the two azido groups has been reported to demonstrate an increased reactivity of the second azido group.55,56 One-pot CuAAC homopolymerization of this AB2 monomer at molar ratios of [AB2]0:[CuSO4·5H2O]0:[ascorbic acid]0 = 90:1:5 reached 96% AB2 conversion at 4 h (Figure 1A) and produced a hyperbranched polymer with an absolute number-average molecular weight Mn,MALLS = 31.7 × 103 (using a multiangle laser light scattering (MALLS) detector coupling with size exclusion chromatography (SEC)) and polydispersity Mw/Mn = 1.36 (based on linear poly(methyl methacrylate) standards in RI detector). However, any attempt of using less Cu catalyst at the ratio of [AB2]0:[Cu]0 ≥ 200:1 failed to B

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Figure 2. (A) 1H NMR spectrum with representative branching structure of purified hyperbranched polymer at 99% monomer conversion and (B) DB evolution as a function of monomer conversions in polymerization of [AB2]0:[B3]0:[CuSO4·5H2O]0:[ascorbic acid]0 = 90:0.1:1:5 in DMF at 45 °C and [AB2]0 = 0.5 mol L−1.

determined by their reactivity difference. At the complete conversion of A1 molecules, the proton nuclear magnetic resonance (1H NMR) spectrum of the crude final product showed that the conversions of azido groups from B2 and B3 were 13% and 89%, respectively, indicating ca. 7 times higher reactivity of the azido groups in B3 than those in AB2 monomers. It is important to note that the higher reactivity of azido groups in core molecules can only achieve fast consumption of B3 in the initial stage. After the reaction of B3 molecules with AB2 monomers, the azido groups in the produced B3containing oligomers carry terminal B groups originated from the reacted monomers. In order to continuously favor the polymer−monomer reactions over the monomer−monomer reactions throughout the polymerization, a strategy to achieve higher reactivity of azido groups in any B3-containing polymers has to be developed. In other words, the higher reactivity of azido groups in B3 core molecules should be relayed to the next-generation B3-containing polymers following a feature of consecutive activation of B group in the core.45,59 To demonstrate this feature in our present system, a second model reaction was carried out using a mixture of monoalkyne A1′ (designed for better resolution in the product characterization), B2, and a B3-derived first-generation dendrimer G1 (comprising six azido groups) at ratios of [A1′]0:[B2]0:[G1]0 = 6:3:1. Since the methylene protons next to the azido groups in both B2 and G1 have similar chemical shift before and after reaction, the use of 1H NMR spectroscopy failed to monitor the conversions of these two azido groups. Instead, the use of A1′ molecules that carried an oligo(ethylene glycol) chain allowed to determine the conversion of B2 and G1 in the reaction using THF SEC with well-resolved elution peaks between B2 and G1 (Figure S7). Using reaction solvent DMF as an internal standard in the SEC characterization, the reaction mixture indicated a 22% B2 conversion when all A1′ molecules were consumed. In contrast, the G1 peak almost disappeared, indicating a retained high reactivity difference. The structure of hyperbranched polymers was determined using 1H NMR spectroscopy with the assistance of twodimensional rotating frame nuclear Overhauser effect spectroscopy (2D ROESY) experiment for peak assignments (Figure S8).60 A typical 1H NMR spectrum in Figure 2A shows wellresolved signals from protons in or adjacent to the triazole rings in the dendritic (D) and linear (L) units. The corresponding DB was calculated to be 0.86 by using the equation DB = 2D/ (2D + L),61 which was significantly higher than the theoretical

further increase the polymer molecular weight (Figure S2). Instead, the polymerization suffered from slower reaction rate (40% AB2 conversion at 4 h), and the produced polymer showed broader molecular weight distribution (Mw/Mn = 1.57), mainly due to the side reactions when the Cu concentration was too low.56 With an aim to produce hyperbranched polymers with high molecular weight in a one-pot synthesis, the present study circumvented the dependence of polymer molecular weight on the amount of Cu catalyst by using the B3 core molecule (Scheme 1). Figure 1A indicates that addition of 0.1 equiv of B3 molecules in the polymerization at molar ratios of [AB2]0:[B3]0:[CuSO4· 5H2O]0:[ascorbic acid]0 = 90:0.1:1:5 resulted in a faster polymerization and produced hyperbranched polymers with higher molecular weight (Figure 1B). The molecular weight increased linearly with the AB2 conversion, and the final product at ∼100% AB2 conversion showed a Mn,MALLS = 335.8 × 103 (theoretical molecular weight at 100% conversion: Mn,theor = ([AB2]0/[B3]0) × FWAB2 = 367.9 × 103) with polydispersity Mw/Mn = 1.06. The SEC results in Figure 1C show monomodal elution peaks with clean shift to highmolecular-weight direction. In contrast, the use of a normal triazido B*3 core molecule (Scheme 1) at the same ratios of [AB2]0:[B*3]0:[Cu]0 = 90:0.1:1 produced hyperbranched polymer with no difference as those synthesized without the use of core molecules (e.g., overlapped Mn,MALLS ∼ conversion curves in Figure 1B and overlapped SEC curves in Figure 1D). The molecular weight difference between the hyperbranched polymers produced from the B3 and B*3 was due to the reactivity difference of the azido groups in these two molecules. The B*3 molecule carried azido groups that had the similar reactivity as those in AB2 monomer. At the initial ratio of [AB2]0:[B*3]0 = 90:0.1 (i.e., 900:1), the possibility of monomer−core reaction was much lower than the monomer−monomer reaction, resulting in low incorporation of B*3 molecules into polymers even at 96% AB2 conversion. As comparison, the B3 core molecule contained a tris-triazoleamine moiety in the structure, which could complex CuI and increased the reactivity of its azido groups54,57,58 to ensure a fast incorporation of B3 molecules into the polymers. To explore the reactivity difference between the azido groups from the B3 core and the AB2 monomer, a model reaction using a mixture of monoalkyne (A1), diazide (B2) and B3 molecules at ratios of [A1]0:[B2]0:[B3]0 = 3:1.5:1 was set up (Figure S6). Since the azido groups from B2 and B3 had the same concentration in the feed, their conversions during the CuAAC reaction were C

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Figure 3. (A) Kinetics and (B) stacked SEC traces (RI signal); (C) number-average molecular weights (Mn,MALLS) and polydispersity (Mw/Mn) of hyperbranched polymer in one-pot polymerizations of AB2 and B3 at various feed ratios of [AB2]0:[B3]0:[CuSO4·5H2O]0:[ascorbic acid]0 = 900:1:x:5x in DMF at 45 °C, where x = 1, 2, 5, 10, 20 and [AB2]0 = 0.5 mol L−1; (D) DB of the purified hyperbranched polymers.

value of DB ∼ 0.5 when assuming all the B groups share the same reactivity.37,62 Furthermore, the samples withdrawn from the reaction mixture at different conversions (27−99%) exhibited almost constant DB = 0.84−0.86 (Figure 2B), supporting the claim that the dangling azido group in the L unit had higher reactivity than the azido groups in T units and monomers.54−56 The effect of Cu amount was investigated by comparing a series of polymerizations under conditions of [AB2]0:[B3]0: [CuSO4·5H2O]0:[ascorbic acid]0 = 900:1:x:5x with x = 1−20 (Figure 3A). At [Cu]0:[B3]0 = 1, the polymerization showed slow kinetics with 86% AB2 conversion after 100 h, largely because the polymer−monomer reactions relied on the dynamic ligand exchange of CuI between the central B3 core and the in situ formed polytriazole structural units (Figure S10) to deliver the catalyst to every azido reactive group in the polymer. It is worth noting that this dynamic ligand exchange is attributing to the higher reactivity of azido groups on B3containing polymer than those on AB2 monomer, as discussed above. In comparison, the use of excess Cu catalyst at [Cu]0: [B3]0 > 1 resulted in faster polymerization since the additional CuI catalyst in the system could freely hop between the polytriazoles within the hyperbranched polymer to catalyze the CuAAC reactions around the periphery of the hyperbranched molecule. For example, the polymerization with x = 10 achieved complete AB2 conversion within 40 min. When more than 10 equiv of Cu catalyst was used (e.g., x = 20), there was no significant change in the polymerization rate because the added Cu catalyst exceeded its solubility in DMF and partially precipitated out.56 On the other hand, the molecular weight of hyperbranched polymers was still governed by the ratio of [AB2]0:[B3]0 and little affected by the amount of Cu catalyst (Figure 3B). At the end of polymerization, all produced hyperbranched polymers showed overlapped SEC elution

chromatograms with almost constant molecular weights. The slightly increased values of Mn,MALLS versus the ratio of [Cu]0: [B3]0 was the result of incomplete monomer conversion and intramolecular reactions in the presence of tris-triazoleamine ligand.63 The use of [Cu]0:[B3]0 > 1 not only increased the polymerization rate but also allowed faster conversion of L unit (reaction of the dangling azido group) to D unit. The DB of hyperbranched polymers increased from 0.65 to 0.86 when more CuI catalyst was used (Figure 3C), evidenced by a significant decrease of L unit signal in the 1H NMR spectra (Figure S11). Depending on the ratio of CuI/B3 complex to the free CuI in the system, it is expected that the DB of hyperbranched polymers can be continuously tuned within the range of 0.65−0.86 by varying the ratio of [Cu]0:[B3]0 in the feed.15 It is interesting to note that the hyperbranched polymers produced in the homopolymerization of AB2 showed the highest DB = 0.91 than any polymers containing the B3 core. This difference was related to the intrinsic property of CuI/tris-triazoleamine catalyst, which was first reported in the CuAAC reaction of diazide and phenylacetylene using tris[(1benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA), a structural analogue of B3 core, as ligand.55 The presence of CuI/tristriazoleamine catalyst decreased the reactivity of the azido group in L units and lowered the DB of hyperbranched polymers. At the same time, the DB of hyperbranched polymers was not affected by the initial molar ratios of [AB2]0:[Cu]0 and [AB2]0:[B3]0, suggesting the independent tunability of DB and molecular weights in one polymerization system. In the presence of B3 core, tunable molecular weights were easily achieved by varying the monomer to core ratios. Polymerizations with various [AB2]0:[B3]0 ratios were carried out at the conditions [AB2]0:[B3]0:[CuSO4·5H2O]0:[ascorbic acid]0 = y:1:10:50, y = 100, 300, 900, and 2700 with D

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Figure 4. (A) SEC traces (RI signal) of hyperbranched polymers produced at various feed ratios of [AB2]0:[B3]0:[CuSO4·5H2O]0:[ascorbic acid]0 = y:1:10:50 in one-pot in DMF at 45 °C, [AB2]0 = 0.5 mol L−1, where y = 100, 300, 900, 2700 (solid lines), and sequential additions of four batches of AB2 monomers with initial condition [AB2]0:[B3]0:[CuSO4·5H2O]0:[ascorbic acid]0 = 100:1:10:50 in DMF at 45 °C and the 2nd, 3rd, and 4th batches of 200, 600, and 1800 equiv of AB2 monomers to initial B3 core (dashed lines). Each batch of AB2 was added at >98% monomer conversion of previous batch without any purification with [AB2]0 kept at 0.5 mol L−1 during the whole reaction. (B) The Mn,MALLS and Mw/Mn as a function of[AB2]total:[B3]0. (C) Hydrodynamic diameter (Dh) (in THF) of hyperbranched polymers produced in one-pot polymerizations of AB2 and B3 at various feed ratios; all of the coefficients of variation (CV) < 0.08. (D) TEM image of the hyperbranched polymer ([AB2]0:[B3]0 = 900:1) on a carbon-coated copper grid (mean diameter ca. 9 nm) with a zoom-in image at top right.

concentration of [AB2]0 = 0.5 mol L−1. All these reactions observed complete monomer conversion within 4 h, producing hyperbranched polymers with sharp monomodal SEC elution peaks (Mw/Mn < 1.08, Figure 4A). The Mn,MALLS of the hyperbranched polymers increased linearly with the [AB2]0: [B3]0 ratio, showing little deviation to the theoretical values (Figure 4B). After polymerization, the Cu catalyst in the hyperbranched polymers could be easily removed via addition of 2 equiv of N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA) to Cu amount to extract all Cu out, producing a colorless product after passing through a neutral alumina column (Figure S12). The one-pot polymerization at very high ratio of [AB2]0:[B3]0 = 2700:1 produced hyperbranched polymers with Mn,MALLS = 1.01 × 106 and polydispersity Mw/ Mn = 1.05. Accordingly, these polymers with various molecular weights exhibited different hydrodynamic diameters in THF (Dh = 6.5−25.1 nm) with narrow distribution, determined by dynamic light scattering (DLS, Figure 4C). These results confirmed that there was one B3 core per hyperbranched polymer, and the polymer molecular weight was directly governed by the amount of B3 core in the initial feed. To visualize individual hyperbranched polymers, the product from the one-pot polymerization with ([AB2]0:[B3]0:[Cu]0 = 900:1:10 was surface-modified by reacting with alkynylated poly(ethylene glycol) (PEG-alkyne, Mn ≈ 688) to avoid aggregation of polymers on TEM Cu grid. A typical transmission electron microscopy (TEM) image of the nonstained PEG-coated polymer is shown in Figure 4D, in

which the PEG arms were invisible and the dark contrast was generated by the electron-dense polytriazole cores. The polytriazole hyperbranched polymers in TEM exhibited uniform size distribution with smaller size (mean diameter ca. 9 nm), as compared to the Dh = 15.4 nm in THF. To further support the selective polymer−monomer reactions throughout the polymerization, sequential monomer addition in a one-pot polymerization was carried out for continuous chain extension. For instance, hyperbranched polymers produced from the ratios of [AB2]0:[B3]0:[CuSO4· 5H2O]0:[ascorbic acid]0 = 100:1:10:50 could be chainextended by addition of three batches of AB2 monomers at 200, 600, and 1800 equiv amounts of the initially added B3 core, respectively. The monomers after each addition reached complete conversion and the produced hyperbranched polymers exhibited overlapped SEC curves as those prepared in the one-pot procedures (Figure 4A), proving there was no or little free CuI catalyst in solution before the addition of each subsequent batch of AB2 monomer. Such consistent control of polymer molecular weights from both the one-pot polymerization and the multibatch procedures demonstrated the robustness of this facile polymerization technique.

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CONCLUSION A novel triazido B3 core molecule that contained a tristriazoleamine moiety was successfully applied for synthesis of hyperbranched polymers with very high molecular weight (Mn > 106), low polydispersity (Mw/Mn < 1.1), and high DB = E

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0.65−0.86 in the one-pot CuAAC polymerization of AB2 monomer. The complexation of B3 core molecule with CuI catalyst activated the azido groups in the B3 molecule and the B3-containing polymers, ensuring faster polymer−monomer reactions than monomer−monomer reactions. The molecular weights of hyperbranched polymers were solely determined by the molar ratio of [AB2]0:[B3]0 in the feed and showed little deviation from the theoretical values. On the other hand, a higher ratio of [CuI]0:[B3]0 significantly accelerated the polymerization and increased the DB of hyperbranched polymers. These results represent the first report on one-pot synthesis of hyperbranched polymers with ultrahigh molecular weight and well-defined structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02678. Detailed experimental procedures and additional characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.G.). Author Contributions †

X.C. and Y.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the ARO YIP award (W911NF-14-1-0227) and ACS Petroleum Research Fund (PRF #54298-DN17) for financial support. H. Gao thanks the startup support from the University of Notre Dame and the Center for Sustainable Energy at Notre Dame.

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DOI: 10.1021/acs.macromol.5b02678 Macromolecules XXXX, XXX, XXX−XXX